IO/configuration: XML/JSON update dependencies

[roccoarpa]

Consider to use the more portable boost xml/json interfaces (rapidxml/rapidjson) to get rid of the actual libxml2/rapidjson dependencies.
In particular, libxml2 is uber-supported on Linux systems, but quite rare on Windows native ones.

Understand if it's more convenient to switch to the newest and drop the older ones, or to keep both for a whhle for a safe redundancy.